Design and optimisation of orbits in proximity of asteroids using solar sails

对小行星（特别是近地小行星）进行近距离观测十分重要，这既是出于科学原因（许多特征今天基本上是未知的，而且小行星彼此之间差异很大），也是出于行星保护和未来对其资源（金属、水）的开发。一种可能的访问多个小行星的方法是使用太阳帆航天器。太阳帆是一种大型、反射性和轻质的薄膜，从航天器上展开，通过反射来自太阳的光子提供推力。这是很有吸引力的，因为它产生一个小的，但连续的，随着时间的推移，没有推进剂的消耗，使高v任务。在这个小组的研究表明，太阳帆的潜力，使任务可以访问多达五个近地小行星在十年内。此外，还发现了各种近地小行星组合的轨迹以及发射日期，这表明了这类推进器的灵活性。虽然行星际之旅是可能的，但进一步的研究挑战是如何围绕小行星（或其附近）：该博士将研究小行星附近的动力学和轨道，以及行星际腿的转移。小行星轨道具有挑战性，原因有几个：小行星是高度不规则的天体，它们的形状和密度事先不知道;此外，大多数是翻滚的，产生不规则和随时间变化的重力场。另一个挑战来自太阳帆的使用，其加速度只能通过姿态机动在一定限度和限制内加以控制，将使用多体和不规则体动力学。最初的办法很可能将以能量水平和零速度曲线为基础，以确保小行星附近的轨道有界。进一步的研究将涉及数值优化，以最大限度地提高科学回报，可靠性和/或其他价值指数的特定轨迹为目标。最终目标是设计和优化完整的轨道，从行星际阶段开始注入小行星轨道，围绕小行星轨道运行所需的时间，并最终进入下一次行星际转移。

Close-up observations of asteroids (particularly near-Earth asteroids, or NEAs) are important both for scientific reasons (many characteristics are largely unknown today, and asteroids are very different one another), for planetary protection and for future exploitation of their resources (metals, water). A possible way to visit multiple asteroids is with a solar sail spacecraft. A solar sail is a large, reflective and lightweight membrane that is deployed from a spacecraft and provides a thrust by reflecting the photons from the sun. This is appealing because it generates a small, but continuous, acceleration over time without propellant expenditure, enabling high-v missions.Research in this group has shown the potential of solar sails to enable missions that can visit up to five near-Earth asteroids within ten years. In addition, trajectories were found for a variety of combinations of NEAs, as well as launch dates, demonstrating the flexibility offered by this type of propulsion. While the interplanetary journey is possible, a further research challenge is how to orbit around (or in proximity of) the asteroids: this PhD will investigate the dynamics and trajectories in proximity of the asteroids, together with the transfer to/from the interplanetary legs.Asteroid orbits are challenging for several reasons: asteroids are highly irregular bodies, and their shape and density are not known in advance; in addition, most are tumbling, generating an irregular and time-varying gravity field. An additional challenge comes from the use of the solar sail, whose acceleration can only be controlled, through attitude manoeuvres, within certain limits and constraints.Multi- and irregular body dynamics will be used. It is likely that an initial approach will be based on the energy levels and zero-velocity curves, to ensure bounded trajectories near the asteroid. Further research will involve numerical optimisation to target specific trajectories that maximise scientific return, reliability, and/or other merit figures. The ultimate goal is to design and optimise complete trajectories that inject into asteroid orbit starting from the interplanetary phase, orbit around the asteroid for a desired amount of time, and eventually depart into the next interplanetary transfer.